Electrical equipment, and in particular electrical equipment operating using alternating current, is subject to varying input signals and conditions. In typical arrangements, alternating current devices in the United States expect to receive a 60 Hz power line source (or 50 Hz in Europe) having a predetermined magnitude (e.g., 120 Volts in North America or 240 Volts in Europe). Although these power sources may vary somewhat, devices made for use with a particular current can typically handle some slight variation in the power signal received.
In some cases, a power signal can vary widely due to external conditions or harmonics. External conditions that may cause harmonics or quasi-direct currents (DC) on a power signal include geomagnetic storms or effects of electrical equipment. Such events can cause the input voltage and current (and resulting power) of a power signal to vary dramatically, causing a potential for damage to the electrical equipment receiving that power signal. Geomagnetic storms or the E3 pulse associated with a high altitude electromagnetic pulse (HEMP) can induce DC or quasi-DC currents called Geomagnetic Induced Currents (GIC) in high voltage power generation, transmission, and distribution system components, i.e. power transmission lines and power transformers. These DC currents can cause half cycle saturation in power transformer cores which in turn can result in excessive reactive power losses, heating, damage and/or failure of such a transformer, particularly in older or poorly maintained transformers. In addition, the half cycle saturation can cause the generation of harmonics of the primary frequency (50 or 60 Hz). This harmonic content can cause power system relays to trigger, which can decouple required compensation components. This in turn can result in the collapse of local or wide area portions of a power grid.
Over approximately the last two decades, several suggested approaches for reducing GIC or HEMP (E3) induced currents in power systems have been proposed. These solutions generally take one of a few forms. A first class of solutions uses a capacitive circuit to simultaneously provide the AC grounding path and a block for the induced DC currents. These solutions generally include a set of switches that allow switching between a normal grounded transformer connection and grounding through the capacitive circuit. These solutions can allow for unintentionally open grounding connections to the transformer neutral, or require expensive electronics for handling ground fault conditions. These capacitive circuit solutions may require readjustment of power system relay settings, as compared to current operational parameters.
A second class of solutions generally includes the continuous use of active components used to reduce potentially damaging GIC events from DC or quasi DC currents in the transformer neutral to ground connection. These solutions typically require expensive power equipment, and are constantly active, such that any failure would render these systems unreliable. Additionally, when this solution is initially installed in the power system many relays/breakers would require readjustments of their settings.
A third class of solutions generally uses a resistive approach in which fixed value resistors are used to continuously reduce the DC current in the neutral to ground connection of a transformer; however in these approaches, the resistor typically must have a high resistance value and would only reduce, not eliminate the DC or quasi DC neutral current. Additionally, during the installation of these classes of solutions a readjustment of the power system's relay settings may be required. As such, there exists no solution that provides a reliable, low cost protection circuit compatible with current power delivery systems. Furthermore, there exists no reliable, testable system for controlling such a protection system that would not require substantial on-site maintenance.
Several suggested approaches for reducing or blocking GIC or E3 induced currents in power systems have been proposed. However, none of these systems provides a comprehensive arrangement for addressing the various types of potentially harmful decisions that may occur. In particular, there has been no known approach that uses a sensing and control system that first senses the presence of GIC or E3 events then switches a DC blocking device to protect high voltage transformers.
For these and other reasons, improvements are desirable.